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Hawaiian Islands Synoptic Discussion and Guidance

814
FXHW60 PHFO 191359
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
359 AM HST Sat Jun 19 2021

.SYNOPSIS...
Moderate trade winds will continue today, then the trades will
become light to moderate next week. An upper-level low just north
of the islands may cause a few heavier showers to develop today.
More stable atmospheric conditions are expected to result in a
relatively dry trade wind weather pattern from Sunday through
most of next week. The weaker background flow will also allow
sea breezes to develop each afternoon, followed by land breezes
at night.

&&

.DISCUSSION...
A 1030 mb surface high pressure system is centered near 39N 141W,
or about 1600 miles northeast of Hilo. A surface ridge extends
west-southwest from this high through a point about 620 miles
north of Lihue. The pressure gradient south and southwest of these
surface features is maintaining moderate trade winds across the
state early this morning. Aloft, loops of water vapor satellite
imagery show a broad upper-level cold core low pressure system is
centered near 24.5N 157.5W, or about 225 miles north of Honolulu.
The close proximity of the cold air aloft associated with this
feature to the state is causing some atmospheric instability.
Satellite imagery and radar reflectivity data show scattered low
clouds with isolated embedded showers moving toward windward
Oahu...Molokai...Maui and the Big Island. There are also a few
clouds and showers moving toward windward Kauai. In addition to
the low clouds, there are cirrus clouds moving up from the
southwest over portions of the Big Island.

The moderate trade wind regime will persist into this evening.
The upper-level low is expected to drift slowly eastward through
this evening. This will keep slightly unstable conditions over
the state. The instability will allow for some enhancement of the
clouds and showers transported by the trades. There is also a
chance of locally heavy downpours if towering cumulus clouds
develop over some of the islands. In addition, there is a
possibility of a few thunderstorms occurring over the upslope
areas of Big Island this afternoon. By late tonight, the upper-
level low is expected to lift out northeast away from the area.
The forecast guidance indicates a ridge will build in aloft over
the region from Sunday through most of next week, which will
cause the atmosphere to become stable.

The forecast models keep light to moderate trade winds in the
forecast from Sunday through the middle of next week. The
relatively weak trade wind regime may allow alternating local
afternoon sea breezes and nighttime land breezes to develop over
each of the individual islands next week. The sea breezes could
result in some clouds forming over leeward and interior sections
of the islands each afternoon. However, the stable and dry
atmospheric conditions will likely limit afternoon shower
coverage. The land breezes, which will initially become
established each evening, will tend to clear skies over most of
the islands at night.

&&

.AVIATION...
A high far northeast of the area will sustain moderate trade winds
through today and beyond. Any showers that develop will mainly
affect windward and mauka sections, with a slight chance of a
heavy shower or two, and even a thunderstorm, through the day as
an upper air disturbance influences the local weather. Overall,
VFR conditions will predominate across the island chain.

No AIRMETs are in effect, and none are anticipated through the
next 24 hours.

&&

.MARINE...
A 1030 mb surface high is centered approximately 1,700 nm
northeast of Honolulu with the associated upper ridge axis about
600 nm north of the state. As ridging holds its ground through
the weekend, trade winds will generally remain moderate the next
several days...locally stronger through the bays and channels
through Sunday. A couple of northern Pacific cold fronts will
pass over the ridge with one this weekend and a secondary front
passage the middle of next week. These fronts may push the ridge
closer to the islands and slightly weaken winds even further by
Wednesday. Gentle to moderate trades will generally maintain seas
in the 4 to 6 foot range through next week.

A series of long period south swells will be moving through into
early next week and this will increase south facing shore surf
heights. This surf may peak near High Surf Advisory levels Sunday,
with a gradual fall from Monday into the middle of next week.
North facing shore surf will remain nearly flat this weekend. A
small bump in north and west facing shore surf is expected from
Sunday night through mid-week as a small west northwest swell
moves into the region. Weak trade winds will keep short period,
wind wave heights small along eastern exposed shorelines through
next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Houston
AVIATION...Kinel
MARINE...Blood

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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